Inkjet recording process has been in rapid progress in various fields, because high-definition images can be recorded in a relatively simple apparatus. Applications of such process have also been diversified, and thus various recording media or inks suitable for respective applications are selected. In recent years, application of the inkjet recording process to industrial fields is under study, and in particular, the development is advanced to provide printers having performances satisfying the requirements of an on-demand type application of carrying out facilitated printing.
The process and the ink used for recording are also studied from various points. For example, an inkjet recording inks, that are hardened when they are exposed to an ultraviolet ray, are in development. Such an ink has a significantly shortened image-fixing period and improved fixing efficiency, compared to inks that should be air-dried or left for penetration into the substrate. However, there is still a need for further improvement of hardening velocity and reduction of hardening energy. It is important in industrial application that the image is brilliant and highly vivid, and also, that a great number of prints are formed in an extremely short period of time at low energy consumption.
As to the UV-hardening inks as described above, hardening inks using a cationically-polymerizable compound mainly based on acrylic compositions are studied (see e.g., JP-A-2002-188025 (“JP-A” means unexamined published Japanese patent application) and JP-A-2005-179506). Further, reduction in the size of pigment particles contained in an ink is attempted for improvement of image brilliance. However, JP-A-2002-188025 does not disclose the diameter of the pigment particles used in practice, and the diameter of the particles is 120 nm or more, even if the smallest one, for example, specifically disclosed in JP-A-2005-179506.
On the other hand, as an aqueous ink which does not demand hardening by ultraviolet ray or the like, it is disclosed that an ink containing pigment particles made in a smaller diameter and in a narrower particle diameter distribution for improving a bronzing property, a glossiness property, and others (see, e.g., JP-A-2003-113341 and JP-A-2003-128955). When used in such an aqueous ink, for example, the pigment fine particles having a particle diameter of 30 nm or less, as disclosed in the Patent Documents above, can be utilized in a stabilized state. However, it is not obvious whether such a nonaqueous hardening ink can show favorable ink properties, while preserving its resistance properties such as a light resistance and a heat resistance sufficiently, to be used as an ink which is fixed to be an image by irradiation of an activating energy ray such as ultraviolet ray or by heating. Furthermore, when the pigment fine particles are applied to a nonaqueous ink, it is not obvious whether or not the pigment fine particles can be dispersed in the ink medium thereof, while keeping the organic pigment fine particles fine.
Considering the requirements in industrial application, particularly the extremely severe requirements demanded recently for nonaqueous inks, conventional inks are still unsatisfactory. For example, further improvement of ink hardening velocity, hardening efficiency, and the like by photoirradiation is desired. In addition, pulverization of pigment fine particles for improvement in transparency was accompanied by a problem of deterioration in light resistance. Thus, it was difficult to obtain inks favorable in both of the transparency and the light resistance.